1. Field of the Invention
The present invention relates to a liquid crystal molecule, and to a liquid crystal display device and a liquid crystal spatial light modulation device that use the liquid crystal molecule.
2. Description of the Related Art
In recent years, active-matrix liquid crystal display devices (LCDs) such as thin film transistor (TFT) devices have come to be widely used in devices from small mobile units to large television sets. LCDs have been enhanced in terms of response speed by adopting techniques such as the impulse driving method. However, the LCDs are still inferior to plasma display panels (PDPs), field emission displays (FEDs), and the like in display quality regarding moving images due to, for example, moving-image blur arising from the slow response speed of the liquid crystal material itself.
Attempts have been made to enhance the speed from the current 60 Hz frame rate drive to 120 Hz or 240 Hz (high frame rate drive) and to enhance the moving-image display quality. The moving-image display quality of LCDs mostly depends on the response characteristics of the liquid crystal material itself, though partly depending on the drive system including TFTs. In other words, the above-mentioned problem is not fundamentally solved and the high frame rate drive is not realized unless the liquid crystal material has high-speed response.
Thus, there is a keen demand for a liquid crystal material that can support a high frame rate drive and exhibit high-speed response allowing the realization of a high moving-image display quality.
Examples of the liquid crystal that can achieve high-speed response include a nematic liquid crystal with a flexoelectric effect, a ferroelectric liquid crystal, and an antiferroelectric liquid crystal. The inventor of the present invention has paid attention to an electroclinic effect in a smectic A phase.
The electroclinic effect is a phenomenon in which the optical axis of liquid crystal molecules (longitudinal axis of liquid crystal molecules) uniaxially aligned in a smectic A phase is inclined in accordance with the intensity of an electric field applied (refer to Garoff et al., Physical Review Letters, Vol. 38, 1977, p. 848). When this type of cell is disposed between polarizing plates orthogonal to each other, transmitted light amount that is dependent on the angle (tilt angle) between the optical axes of the polarizing plate and the liquid crystal is obtained (refer to Formula (A)), and the maximum transmittance is obtained at a tilt angle of ±45°.T/T0=sin2(2θ)×sin2(πΔnd/λ)  (A)where T is transmitted light amount, T0 is incident light amount, θ is the angle (tilt angle) between the optical axes of the polarizing plate and the liquid crystal, Δn is the birefringence of the liquid crystal, d is the thickness of the liquid crystal layer, and λ is the wavelength of the transmitted light.
The dependence of transmittance on tilt angle in the case of a retardation (=Δnd) giving the maximum transmittance is calculated using Formula (A). FIG. 5 shows the result.
The response time in the electroclinic effect is as short as several microseconds to several tens of microseconds. Furthermore, it is advantageous that the inclination angle (tilt angle) of an optical axis is proportional to electric field intensity (i.e., voltage modulation of transmitted light is possible) when the electric field intensity is low. In other words, this is a display mode particularly suited to an active matrix drive.
However, the tilt angle in the electroclinic effect produced from existing liquid crystal materials is not so large, and sufficient optical modulation is not obtained.
Examples of a liquid crystal material that shows a large tilt angle include materials in which a siloxane is added to an achiral terminal end. This may be because the addition of a siloxane, which is a functional group that is larger in volume than ordinary alkyl chains and is flexible, to a terminal group of a molecule renders a core portion of the molecule more easily movable under the application of an electric field, the core portion contributing to optical modulation. According to Naciri et al., Chem. Mater. 1995, 7, pp. 1397-1402, liquid crystal molecules having a structure in which a siloxane is added to an achiral terminal end have a maximum tilt angle of 26°. However, the transmittance calculated from Formula (A) is about 60% at most, and the polarization is high because such a liquid crystal molecule has a nitro group as a polarization group. The transmittance is still insufficient in consideration of the practical use of display devices or the like.
Japanese Unexamined Patent Application Publication No. 2008-150334 discloses a liquid crystal molecule having an asymmetric carbon atom, and having a core portion with a terminal phenyl group and an organosiloxane that are opposite each other so as to sandwich the asymmetric carbon atom. However, since the terminal end of the liquid crystal molecule is an alkoxy-substituted phenyl group that does not have a functional group, such a liquid crystal molecule is not satisfactory in terms of a small tilt angle. Therefore, it is desired to further increase a tilt angle.